Packaged food products are commonly available to the public in vacuum sealed airtight packets, such as those made of pliable plastic films. The packets are termed airtight to the extent that the films are relatively impermeable to air, so that the components sealed within them at the time of preparation remain largely anaerobic. Such products must be prepared free of pathogenic organisms, especially toxin-producing anaerobes. Pathogenic organisms that may contaminate packaged food products include, by way of nonlimiting example, Clostridium botulinum, C. perfringens (Lucke et al., in “Ecology and Control Foods” (A. H. W. Hauschild and K. L. Dodds, eds.) Marcel Dekker, New York, 1993, pp. 177–207; Smart et al., J. Appl. Bacteriol. 46, 377–383 (1979); Roberts et al., J. Fd. Technol., 14, 211–226 (1979); Tompkin, Food Technology, 34, 229–236, and 257 (1980); Bryan et al., Amer. Public Health, 61, 1869–1885 (1971); Microbial Ecology of Food Commodities—Microorganisms in Foods 6; Blackie Academic and Professional, 1998, p. 115), Listeria monocytogenes, Escherichia coli, Bacillus cereus, Enterococcus faecalis, and similar microorganisms. Among these, spore-forming, toxin-producing microorganisms are of particular concern, because any spores produced by viable cells may survive and grow to produce toxins subsequent to manufacturing or domestic heating steps. Such microorganisms include species of the genus Clostridium. 
In U.S. Pat. Nos. 4,888,191 and 5,017,391, Anders et al. disclose compositions and methods related to the use of lactate salts to delay C. botulinum growth in a foodstuff such as fish or poultry. The foods are heated to a temperature sufficient to cook the meat but not to sterilize the product. Anders et al. suggest that lactate may be used alone, or in combination with other agents such as sodium nitrite. These patents fail to discuss nisin or its properties.
Maas et al. (Appl. Envir. Microbiol., 55, 2226–2229 (1989)) report that lactate, when incorporated into a turkey meat vacuum-packed composition, delays the generation of botulinum toxin in a manner directly dependent on the concentration of lactate introduced into the composition. Maas et al. do not mention nisin.
Nisin is a peptide-like antibacterial substance produced by microorganisms such as Lactococcus lactis subsp. lactis (formerly known as Streptococcus lactis). Its structure is illustrated in U.S. Pat. No. 5,527,505 to Yamauchi et al. The highest activity preparations of nisin contain about 40 million IU per gram. Commercial preparations of nisin are available. For example, one commercial preparation, NISAPLIN™, containing about 1 million IU per gram is available from Aplin & Barrett Ltd., Trowbridge, England; another commercial preparation, CHRISIN™, containing about 1 million IU per gram is available from Chr. Hansen A/S (Denmark). Nisin has no known toxic effects in humans. It is widely used in a variety of prepared dairy foods. Experimental use in preserving other foods has also been reported. Details on these applications are provided below.
A number of efforts have been reported since 1975 directed to reducing uncoupled acid production in dairy fermentations by controlling the post-fermentation acidification of yogurt. In some of these studies, a nisin producing culture was introduced in an attempt to inhibit these effects. Kalra et al. (Indian Journal of Dairy Science, 28: 71–72 (1975)) incorporated the nisin producing culture Streptococcus lactis (now known as L. lactis subsp. lactis) along with the yogurt culture before fermentation. Others introduced nisin in milk prior to fermentation (Bayoumi, Chem. Mikrobiol. Technol. Lebensm., 13:65–69 (1991)) or following fermentation (Gupta et al., Cultured Dairy Products Journal, 23: 17–18 (1988); Gupta et al., Cultured Dairy Products Journal, 23: 9–10 (1989)). In all cases, the rate of post-fermentation acidification was only partially inhibited by these treatments and the yogurt continued to become more acidic throughout its shelf life. However, as pH drops, flavor and texture defects develop.
In U.S. Pat. No. 5,527,505, by Yamauchi et al., yogurt was produced from raw milk by incorporating a nisin-producing strain, Lactococcus lactis subsp. lactis, along with the traditional yogurt culture consisting of Streptococcus salivarius subsp. thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB). Yamauchi et al. teach that the lactococci are needed to secrete the nisin, whose effect is to retard the activity of ST and LB. The resulting yogurt therefore contains the lactococci used to produce the nisin. Nonetheless, the acidity of yogurt containing the nisin-producing bacteria increased by 64 to 96 percent in 14 days, in various experiments inoculated with differing amounts of L. lactis subsp. lactis, compared to the initial acidity at the completion of fermentation. Other studies (Hogarty et al., J. Fd. Prot., 45:1208–1211 (1982); Sadovski et al., XX International Dairy Congress, Vol. E: 542-5-44 (1978)) also noted acid production and development of bitterness at low temperature by some mesophilic starter lactococci in dairy products.
In U.S. Pat. No. 5,015,487 to Collison et al., the use of nisin, as a representative of the class of lanthionine bacteriocins, to control undesirable microorganisms in heat processed meats is disclosed. In tests involving dipping frankfurters in nisin solutions, the growth of L. monocytogenes was effectively inhibited upon storage at 4° C.
Chung et al. (Appl. Envir. Microbiol., 55, 1329–1333 (1989)) report that nisin has an inhibitory effect on gram-positive bacteria, such as L. monocytogenes, Staphylococcus aureus and Streptococcus lactis, but has no such effect on gram-negative bacteria such as Serratia marcescens, Salmonella typhimurium, and Pseudomonas aeruginosa when these microorganisms are attached to meat.
Thomas et al. (J. Food Prot., 65, 1580–1585 (2002) recently demonstrated the use of purified nisin to control spoilage by spore-forming bacteria such as Bacillus and Clostridium in a pasteurized mashed potato product.
Nisin has been added to cheeses to inhibit toxin production by Clostridium botulinum (U.S. Pat. No. 4,584,199 to Taylor). U.S. Pat. No. 4,597,972 to Taylor discloses a detailed example in which chicken frankfurter components are shown to require the presence of both added nitrite and added nisin in order to prevent or delay botulinum toxin production when challenged with C. botulinum. 
Nisaplin™ has been found to preserve salad dressings from microbiological contamination, such as challenge by Lactobacillus brevis subsp. lindneri, for an extended shelf life period (Muriana et al., J. Food Protection, 58:1109–1113 (1995)).
More recently, nisin-containing whey derived from nisin-producing cultures has been used to stabilize a variety of food products. For example, U.S. Pat. No. 6,242,017 (Jun. 5, 2001) and U.S. patent application Ser. No. 09/779,712 (filed Feb. 8, 2001) use nisin-containing whey to stabilize cooked meat as well as cooked meat and vegetable combinations; U.S. patent application Ser. No. 09/779,756 (filed Feb. 8, 2001) uses nisin-containing whey to stabilize cooked pasta compositions; U.S. Pat. No. 6,110,509 (Aug. 29, 2000) uses nisin-containing whey to stabilize cream cheese; U.S. Pat. No. 6,113,954 (Sep. 5, 2000) uses nisin-containing whey to stabilize mayonnaise products; and U.S. Pat. No. 6,136,351 (Oct. 24, 2000) uses nisin-containing whey to stabilize fermented dairy products, all of which are hereby incorporated by reference in their entireties.
U.S. Pat. No. 5,229,154 (Jul. 20, 1993) provides a method for producing fresh, refrigerated mashed potatoes in which the potatoes were overcooked until they did not gel after cooling. The overcooked potatoes were then ground, blended, pasteurized, chilled, injected with an inert gas and then packaged for refrigerated storage. In order to maximize shelf life, the potato mixture was not exposed to the open atmosphere during processing. A shelf life in the range of 6–8 weeks was reported.
U.S. Pat. No. 5,536,525 (Jul. 16, 1996) provides a method for producing frozen, instant mashed potato product which can be dispensed into oven-stable trays using a high-speed frozen food packaging line. A slurry containing a fat-containing ingredient or a water substitute in water is heated to a temperature above the gelatinization temperature of potato starch but below the boiling point of the slurry. The heated slurry is then mixed with dehydrated potato solids in a closed mixing chamber. Hydration of the potato solids is completed under static conditions. The resulting mashed potatoes are then dispensed into the oven-stable trays, packaged, and frozen.
U.S. Pat. No. 6,261,612 (Jul. 17, 2001) provides a frozen mash potato product having, at least in its top portion or surface, a browning agent. The frozen product can be heated in a conventional or microwave oven to provide a golden brown color. Of course, frozen mashed potatoes are more easily stabilized against microbial growth compared to refrigerated; consumers, however, generally view refrigerated products as having a higher quality.
Nisin has also been used as an adjunct to thermal processing to decrease the severity of heat treatment to product a shelf stable cut, canned potato product (Lokshina et al., Trudy-Vsesoyuznyi, 19, 66–67 (1973).
There remains a need for compositions and procedures related to prepared food products, especially starchy food products such as fully hydrated mashed potatoes and fully hydrated potato pieces (i.e., sliced or cut potatoes), that inhibit the growth of pathogenic microorganisms, and the production of toxins by them, using natural or innocuous ingredients. There also remains a need for fully cooked and hydrated mashed potatoes and fully cooked and hydrated potato pieces having a relatively long shelf life under refrigerated conditions. There also remains a need for fully cooked and hydrated mashed potatoes and potato pieces which have a reduced tendency to undergo syneresis during a relatively long shelf life under refrigerated conditions. There also remains a need for fully cooked and hydrated mashed potatoes and potato pieces with a cleaner flavor and better texture than can be achieved by the addition of lactate or reduction of pH. There also remains a need for fully cooked and hydrated mashed potatoes and potato pieces that have a reduced tendency to undergo syneresis during a relatively long shelf life under refrigerated conditions. The present invention addresses these needs.